Capacitive sensors can directly sense electrical fields. Additionally, capacitive sensors can indirectly sense other variables such as pressure associated with a button push. Capacitive sensors are composed of conductive sensing electrodes, a dielectric, and detection circuits that detect changes in capacitance. There are a variety of applications for capacitive sensors including using capacitive sensors as keys or buttons on the keypad of a wireless device.
Like traditional mechanical switches, the effectiveness of a capacitive sensor is dependent on the sensor sensitivity under a variety of different operating conditions. The sensitivity of a capacitive sensor is determined by the physical design, the method used to measure the capacitance and the determination of the capacitive change.
Capacitive sensors may be designed on affordable printed circuit boards (PCBs) such as a standard PCB or printed flex circuits using the same copper material that is used for signal routing. Typically, the sensitivity of the sensor is determined by the physical size of the sensor and a combination of the plastic overlay dielectric constant, including the dissipation factor, and thickness of the overlay material.
Referring to FIG. 1 there is shown an illustrative capacitive sensor or switch that can be used to detect a button being pushed. The capacitive sensor 10 acts as a keyboard button and consists of a first conductive element 12 that is adjacent a second conductive element 14 and separated by a dielectric 16. Generally, there is a small edge-to-edge capacitance associated with dielectric 16.
With respect to button sensitivity, there must be enough sensitivity to detect a button push while avoiding excessive sensitivity that results in false positives due to electromagnetic interference (EMI). Since wireless devices communicate using electromagnetic waves and continuously adjust signal strength, the wireless device generates variable EMI that interferes with the capacitive sensing switch.
Referring to FIG. 2 there is shown the capacitive switch being pushed by a user. When the illustrative finger 20 is placed on the plates 12 and 14 there is an additional capacitance 22 and 24 that increases the overall capacitance and the result is an increase in the RC rise time during the charging cycle. This change in capacitance determines that a button has been pushed.
To detect that a button has been pushed, the change in capacitance is measured. One method for measuring capacitance is applying a constant-current source continuously to charge the capacitive sensor to a reference threshold level on a comparator. The comparator will pulse high each time the capacitive sensor reaches the reference threshold level; this closes the switch and discharges the capacitor and resets the counter. To determine whether a user is in contact with the capacitive sensor the quantity of clock cycles that correspond with the capacitive sensor being charged up to the reference level on the comparator are measured. The number of clock cycles may then be compared with a preset threshold detection setting to determine whether a button has been pushed. An alternative method for measuring capacitance may employ an analog-to-digital converter.
In operation, the illustrative button 10 works on the principle that when a finger 20 is applied to the button, the capacitance increases and lengthens the RC time constant that results in a baseline minimum RC time constant and a “finger present” maximum RC time constant.
With respect to wireless devices, there are various limitations to relying exclusively on the minimum RC time constant and “finger present” maximum RC time constant. One of these limitations is caused by the variable EMI associated with the wireless communications. As the EMI increases and decreases, there is no change to the baseline minimum RC time constant, and there is no change to the “finger present” maximum RC time constant. Additionally, making the capacitive switch button more sensitive does not resolve the variable EMI concerns, because increased sensitivity results in false positives. Thus, there is a need to effectively initialize a capacitive sensing switch on a wireless device.